This invention relates to chip thermistors of the type which are commonly used for the protection of an electronic circuit or as a temperature-detecting sensor and, more particularly, to chip thermistors having electrodes formed overlappingly both on an outer surface of and inside a thermistor element.
The demand to be surface-mountable directly to a circuit board is just as strong on thermistors as on other kinds of electronic components. For this reason, many kinds of thermistors in the form of a chip (or chip thermistors) have been considered. FIG. 8A shows an example of prior art chip thermistor 61 having outer electrodes 63 and 64 formed at both end parts of a thermistor element 62. Each of the outer electrodes 63 and 64 is formed on one of the end surfaces and reaches the four side surfaces adjacent thereto such that the chip thermistor 61 can be surface-mounted, say, by soldering to electrode lands on a printed circuit board.
Inside the thermistor element 62, there may be inner electrodes 65, 66 and 67 each electrically connected to one of the outer electrodes 63 and 64, as shown in FIG. 8B, such that the resistance between the outer electrodes 63 and 64 is determined not only by the specific resistance (or the resistivity) of the thermistor element 62 but also the overlapping areas of the inner electrodes 65-67.
FIG. 8C shows another chip thermistor 68 of a kind having no inner electrodes inside its thermistor element 62. In this case, the resistance between the outer electrodes 63 and 64 is determined by the distance therebetween and the specific resistivity of the thermistor element 62.
FIG. 9 shows still another prior art chip thermistor 71 characterized as having outer electrodes 73 and 74 formed opposite each other on the upper surface of a thermistor element 72 of a semiconductor ceramic material such that they are separated by a specified distance D. In this example, the resistance is adjusted by the distance D of separation between the outer electrodes 73 and 74. Thus, this distance D must be changed for each type or lot of thermistors to be mass-produced, corresponding to the desired resistance. If the desired resistance value is very small, in particular, the distance of separation D must accordingly be made small, but if this distance D is made too small, the two outer electrodes 73 and 74 may contact each other. Since the rate of change in resistance per unit change in distance D becomes large as D is made smaller, it becomes difficult to control the resistance value and hence the variation in the resistance values of the obtained products also becomes large.
With prior art chip thermistors of the types shown in FIGS. 8A, 8B and 8C at 61 and 68, the variation 3.sigma./x (where .sigma. is the standard deviation and x is the average) in the resistance values is fairly large, being about 4-10%. Thus, there has been a strong demand to reduce this variation, say, to within about .+-.1%, but it has been very difficult to respond to this demand. Another problem of this type of prior art chip thermistors was that a fillet is likely to be formed by a solder while it extends upward as it is surface-mounted, say, onto a printed circuit board from the bottom sides 63a and 64a of the outer electrodes 63 and 64 because this would make a high-density mounting difficult. Because of their shape, furthermore, these bottom sides 63a and 64a of the outer electrodes 63 and 64 cannot easily be bonded by a so-called bump-bonding method which is frequently used for effecting a high-density mounting.